328 MEMOIRS OP THE NATIONAL~ACADEMY OF SCIENCES. 



nuclei, and so on until the number is very great. As this process of division goes on the nuclei, 

 each with an investing layer of protoplasm, gradually migrate to the surface of the yolk, and at 

 last form a superficial investing layer around a central yolk, out of which all the protoplasm has 

 been withdrawn. 



The yolk does not divide up into typical yolk pyramids, although the outlines of the blasto- 

 meres are sharply indicated by transitory superficial furrows. 



The embryonic area is soon marked out as a region where the nuclei are densely crowded, and 

 the point of invaginatiou is indicated by a solid ingrowth which penetrates the yolk to form the 

 inner layers of the embryo. The subsequent stages of embryonic development were not followed 

 in detail. 



The larva hatches in the afternoon, and during the following night the parent moults and lays 

 another brood of eggs. 



At the time of its escape the larva is a Protozoea, and its later history is of great interest, since 

 it unites features of resemblance to Lucifer, Sergestes, Peneus, and to the prawns in general, with 

 individual peculiarities in which it differs from all of them. 



At the time of hatching (PI. VII and PL XI, Fig. 25) it has sessile eyes, locomotor antenna?, 

 an enormous mandible, a deeply forked telson, a long rostrum, and a complete series of append- 

 ages as far as the first pereiopods, which are essentially like the third maxillipeds. The long 

 hind body has no appendages and is only vaguely divided into somites. 



Five or six hours after hatching it changes into a true zoea, much like that of an ordinary 

 rnacrouran (PL viii). The carapace becomes much enlarged ; the rostrum is shortened to less than 

 half its former length, the mandible becomes small, the forks disappear from the telsou, the eyes 

 become stalked, the antenna? are shortened like those of a zoea, and the maxillipeds become the 

 chief .locomotor organs. 



As these larva?, could not be reared in captivity the later stages were studied from captive 

 specimens, but Professor Herrick has proved that the Beaufort larva; are either young Stenopi or 

 else the larva? of some closely allied species which is at present unknown. 



A specimen a little older than the oldest Beaufort specimen was captured at Nassau (PL xn). 

 It is in the Mastigopus stage, with greatly elongated eyes, and with antenna? which are gradually 

 approximating to those of the adult. The third maxillipeds are now extremely long and are the 

 largest of all the limbs, while the huge, oar-like fifth pereiopod of the preceding stage is now 

 reduced to a rudimentary bud, and the fourth is also reduced to a two-jointed rudiment. 



It thus appears that, as in the Sergestida?, the last two pairs of " walking legs'' are shed alter 

 the My sis stage, to be again reconstructed in the Mastigopus stage. After several moults the 

 Mastigopus larva gradually assumes the adult form, the principal changes being the shortening 

 of the eyes and the reacquisition of the fourth and fifth pereiopods. 



ALPHEUS. 



The genus Alpheus includes a large number of small, brilliantly colored crayfish-like Cru- 

 stacea, which are widely distributed, although all are essentially tropical. Two species range 

 as far northward as the coast ot Virginia, but the true home of the genus is the warm water 

 between tide-marks or near the shore in coral seas, and they occur in the greatest abundance 

 and variety in all the sounds and inlets among coral islands. They are well adapted, in structure 

 as well as in habits, for a life among the coral, and of all the Crustacea which abound upon the 

 coral reefs the genus Alpheus is one of the most common and most thoroughly characteristic (PL 

 i, n, and iv). 



Nearly every mass of sponge or alga? or of copal rock or living coral which is fished up from 

 the bottom and broken to pieces contains specimens of one or more species of Alpheus, and pieces 

 are often found which fairly swarm with these little animals. 



A few of the species wander over the bottom, and wandering individuals of other species are 

 found occasionally, but their true home is in the fubes of sponges and the holes and crannies in 

 the porous coral limestone, or under the broken shells and fragments of limestone which lie upon 



